FIG. 1 shows a construction of a conventional magnetic bearing control device of this type. An AC power from a commercial power supply 1 is input via a power factor control circuit (PFC) 2 to a DC/DC converter 3 for an inverter and a DC/DC converter 4 for the bearing. An output from the DC/DC converter 3 for the inverter is supplied to an inverter 5, and an output from the inverter 5 is supplied to a motor M. An output from the DC/DC converter 4 for the bearing is supplied to a driver 7 for the bearing and a DC/DC converter 8 for the control power supply. An output from the driver 7 for the bearing is supplied to each excitation coil 9 of at least a pair of opposed magnetic bearing electromagnets (not shown), and an output from the DC/DC converter 8 for the control power supply supplies an appropriate voltage to an electronic circuit (not shown) in the device. Thus, a unit to be suspended (not shown) is suspended by an electromagnetic force, and rotated by a torque of the motor M.
In power failures, to use a regenerative power, a power generated by the motor M controls a frequency of the inverter 5 to control a regenerative voltage to be constant, and is supplied to a backup DC/DC converter 6. An output from the backup DC/DC converter 6 is supplied to the DC/DC converter 8 for the control power supply and the driver 7 for the bearing to decelerate the suspended unit with supported without contact until a safe number of rotation (i.e. a number of rotation that allows safe support by a touchdown bearing) is reached.
When the inverter 5 is operated by a PAM (pulse amplitude modulation) control scheme in the magnetic bearing control device having the above described construction, the DC/DC converter 3 for the inverter supplies a gradually increased voltage from a start to the inverter 5, and the voltage is frequency-converted by the inverter 5 and supplied to the motor M. The PAM control scheme generally produces low noise, but has a high ripple current resulting in problems in efficiency, a torque ripple, or the like. There is also a problem of a low input power factor at low voltages.
When the inverter 5 is operated by a PWM (pulse width modulation) control scheme, the DC/DC converter 3 for the inverter supplies a constant voltage to the inverter 5, and the voltage is pulse-modulated and frequency-converted by the inverter 5 and supplied to the motor M. The PWM control scheme is generally superior in response in accelerating and decelerating operations, but inferior in noise of the motor M to the PAM control scheme.
The magnetic bearing control device having the above-described conventional construction uses a large number of converters for power supply, such as the DC/DC converter 3 for the inverter, the DC/DC converter 4 for the bearing, the backup DC/DC converter 6, and the DC/DC converter 8 for the control power supply, and, thus, there is also a problem that the device has a large size and is expensive and complex.
The present invention has been achieved in view of the above, and an object of the present invention is to provide a magnetic bearing control device that can operate a motor with low noise, has a simple system construction and is compact and inexpensive.